1. Field of the Invention
The present invention relates to a light-emitting compound and an organic electroluminescence device.
2. Description of the Related Art
Recently, organic electroluminescence devices (OLEDs) are attracting an attention as a light-emitting technique for next-generation displays and lighting. At the initial stage of the investigation of OLEDs, fluorescence was mainly used as a light-emitting mechanism in an organic layer. However, it is considered that the upper limit of the internal quantum efficiency of fluorescence is 25% in theory. Actually, there was little report which shows a quantum efficiency of more than 25% in OLEDs using fluorescence. On the other hand, it was shown that an internal quantum efficiency of 100% can be obtained in principle in OLEDs using phosphorescence. This is because phosphorescence enables emission of light from excited triplets whereas fluorescence is emitted from only excited singlets and the excited triplets statically exist by thrice as much as the excited singlets. Actually, an experimental result which shows an internal quantum efficiency of approximately 100% was reported in OLEDs using phosphorescence.
A light-emitting layer using phosphorescence has a mainstream structure in which a host material comprising an organic material is doped with a light-emitting metal complex including iridium or platinum as a central metal. One of the mechanisms of generating an exciton for the light-emitting dopant in the phosphorescence light-emitting layer is as follows. Namely, the host material is excited by injection of electrons and holes from an anode and a cathode, the light-emitting dopant is excited by energy transfer from the host material to the light-emitting dopant, and light is emitted during the energy deactivation process from the excited state to the ground state thereof.
As shown in FIG. 1, the larger the overlap (shown by A in FIG. 1) of the emission spectrum of the host material and the absorption spectrum of the light-emitting dopant is, the better the energy transfer efficiency from the host to the light-emitting dopant is. This is called Foerster's energy transfer mechanism.
Host materials for light-emitting layers using phosphorescence are roughly classified into low molecular-type and high molecular-type. A light-emitting layer comprising a low-molecular host material is mainly formed by vacuum co-evaporation of the low-molecular host material and the light-emitting dopant. A light-emitting layer comprising a high-molecular host material is mainly formed by applying a solution mixed with the high-molecular host material and the light-emitting dopant.
Typical examples of the low-molecular host material include p-biscarbazolylphenylene (CBP). Typical examples of the high-molecular host material include polyvinylcarbazole, referred to as PVK (Jpn. J. Appl. Phys., Vol. 39 (2000) pp. L828-L829, and Adv. Mater., 2006, 18, 948-954). The structure and the numbers for the substitution positions of carbazole, and the structure of PVK are shown below.

Carbazole (the numerals are the numbers for the substitution positions)

The light-emitting dopant materials include blue-emitting dopant materials, green-emitting dopant materials, and red-emitting dopant materials. Typical examples of the blue-emitting dopant materials include bis(2-(4,6-difluorophenyl)pyridinate iridium complex [hereinafter referred to as FIrpic]. Typical examples of the green-emitting dopant materials include tris(2-phenylpyridine)iridium complex [hereinafter referred to as Ir(ppy)3]. Typical examples of the red-emitting dopant materials include Bt2Ir(acac).
Here, formation of a light-emitting layer comprising these light-emitting dopant materials and PVK which is a high-molecular host material is envisaged. The emission wavelength of PVK is 420 nm, whereas the absorption band, which is responsible for emission of the light-emitting dopant material, is 380 nm for FIrpic, the blue-emitting dopant material, 410 nm for Ir(ppy)3, the green-emitting dopant material, and 480 nm for Bt2Ir(acac), the red-emitting dopant material. Therefore, for example, where efficient energy transfer from PVK to FIrpic is desired, it is desirable to shift the emission wavelength of PVK toward shorter wavelengths.
Furthermore, a desirable property of the host material for the light-emitting layer using phosphorescence is that the host material does not deactivate the excited triplet state of the light-emitting dopant. For this purpose, it is desirable that the excited triplet energy of the host material is higher than the excited triplet energy of the light-emitting dopant, and thus it is also desirable to shift the emission wavelength of the host material toward shorter wavelengths.